Impact absorbing macadam

ABSTRACT

An impact absorbing macadam comprises 10 to 75% (preferably 25 to 45%) of a particulate rubber having a particle size of up to 40 mm, 25 to 90% (preferably 45 to 65%) of an aggregate having a particle of up to 40 mm and from 5 to 9% of a polymer modified bituminous binder. Typically such a macadam has a void volume, interconnected or unconnected, of from 5 to 25%. The polymer which modifies the bituminous binder is preferably an unbranched styrene butadiene styrene block copolymer forming about 7% of the modified binder. The macadam is suitable as a base for sports pitches and athletic tracks without a rubber shock pad overlay.

BACKGROUND OF THE INVENTION

The invention relates to an impact absorbing macadam suitable for roads,railways, synthetic sports pitches, atheletics tracks, children's safetysurfaces and other applications.

Macadam, also called flexible pavement, is a combination of aggregate,which may be naturally occurring or may be crushed rock, and a binderwhich is usually a bitumen. It may be produced by raising thetemperature of both components to 150° C.-180° C., and mixing so thatthe binder coats the surface of the aggregate. Alternatively, anemulsion of bitumen in water may be used instead of a hot binder, inwhich case the resulting macadam is referred to as a cold macadam or asemulsion macadam. Macadams are of a granular nature and employpredominantly a single or nominal sized aggregate, the size varyingaccording to the requirement. For instance a macadam may be availablewith nominal aggregate sizes of 3 mm, 6 mm, 10 mm, 14 mm, 20 mm, 28 mmor 40 mm. The nominal sized aggregate may be combined with smaller sizesin varying proportions to provide an open textured, medium textured ordense macadam. The macadam is placed by hand raking or paving machineand is rolled.

The invention provides an impact absorbing macadam comprising from 10 to75% by weight of particulate rubber having a particle size of up to 40mm, from 25 to 90% by weight of an aggregate having a particle size ofup to 40 mm and from 5 to 9% by weight of a polymer modified bituminousbinder.

SUMMARY OF THE INVENTION DETAILED DESCRIPTION OF THE INVENTION

The invention provides an impact absorbing macadam comprising from 10 to75% by weight of particulate rubber having a particle size of up to 40mm, from 25 to 90% by weight of an aggregate having a particle size ofup to 40 mm and from 5 to 9% by weight of a polymer modified bituminousbinder.

The impact absorbing macadam of the invention may have a void volume offrom 5 to 25%, preferably 17%. The voids may be interconnected, that isallowing a natural path of drainage, the preferred range then being from15 to 25%. The voids may alternatively be unconnected, that is having acontinuous particle size distribution approximating to the maximumdensity curve, the preferred range then being from 5 to 20%. The macadammay be formulated so as to be open textured, medium textured or dense.

The particulate rubber may be virgin rubber or may be recycled rubber,for example that obtained by grinding used vehicle tyres. Preferably themacadam contains from 25 to 45% of the particulate rubber. The preferredparticle sizes are up to 15 mm, and most preferably from 4 to 10 mm.Particulate rubber of different particle sizes may be used, butpreferably 10 mm size particles are included.

The aggregate may be any aggregate conventionally used in the productionof macadams, either naturally occurring or an industrial by-product.Coarse aggregates, which are those having particle sizes above 2.36 mm,may be included. These course aggregates are preferably of a particlesize up to 15 mm. Most preferably the coarse aggregates are of sizessimilar to those of the rubber particles used, and macadam according tothe invention may contain more than one particle size, e.g. 10 mmaggregate and 6 mm aggregate. Fine aggregates, also known as sands,which are those having particle sizes above 75 microns but below 2.36mm, may be included in amounts up to 35% by weight of a macadamaccording to the invention. Fillers, which are ultra-fine aggregateshaving particle sizes below 75 microns, must be included and mayconstitute up to 10% by weight of a macadam according to the invention.

The polymer modified bituminous binder comprises an elastomeric polymer,a plastomeric polymer or a mixture thereof in admixture with acompatible bitumen. Suitable polymers include unbranched styrenebutadiene styrene block copolymer and styrene butadiene rubber. Thebinder most suitably constitutes from 6.5 to 7.5% by weight of a macadamaccording to the invention. The polymer preferably constitutes up to 11%by weight, and more preferably from 6 to 8% by weight, of the binder.

Other additives and components conventionally used in macadams, such ascolourings, anti-stripping additives and deferred met agents, e.g.kerosene, may be included in the macadam according to the invention.

The macadam of the invention may be prepared using a conventionalasphalt mixing plant having a separate means to access the particlerubber into the mixer bowl. A dry mix of aggregate and filler ispreheated to 200° C.±20° C. To this is added the rubber particulateeither at ambient temperature or preferably pre-heated to a softeningtemperature dependent on the requirement of the final mix but up to 100°C. Following admixture, the polymer modified bituminous binder at atemperature of from 150° C. to 200° C., preferably 180° C., is added andfurther mixing is effected. The resultant mix can be laid usingconventional plant by hand or placed in moulds and compacted to apredetermined void content. A cold macadam or emullion macadam method ofpreparation may alternatively be used.

It is believed that, when the macadam according to the invention isprepared, a molecular link between the particulate rubber surfaces andthe polymer of the polymer modified bituminous binder is formed. Much ofthe rubber, however, remains solid and by bedding between the crushedrock aggregate particles, provides a shock absorbency to the laid,rubberized macadam. Upon subjection of the macadam, to imposed stress,deformation of the rubber particles occurs. This in turn tensions orcompresses the bitumen. Upon release of the imposed Stress the rubber,which is predominantly elastic, returns to its natural state pulling orpushing the bituminous binder to which it has linked.

It is believed that, when used as a traffic surface or as part of a roadconstruction, loads imposed on the macadam cause the rubber to compress.Release of these loads causes the macadam to revert to an unpressed formthereby reducing wear and potentially increasing the life of the roadsurface.

Conventional macadams do not absorb impact to any high degree. To form asynthetic sports pitch, for example football or hockey, it is necessaryto overlay a macadam base (30 to 40 mm depth, one layer or two layers ofvarying thicknesses) with a rubber shock pad and a long pile (e.g. 23mm) synthetic carpet, the pile of which is filled with sand. The rubbershock pad usually consists of a particulate or granular ruber,preferably styrene-butadiene rubber, formed into a mat by a resinbinder. The shock pad may typically be from 7.5 to 20 mm in thickness.In effect the game is played on sand. The pile of the carpet retains thesand in position, preventing it from heaping up or dispersing. Therubber shock pad provides resilient and impact absorption, enabling therequiremnents of various sporting authorities to be met. The macadamprovides a firm base. A synthetic athletics track is similar inconstruction, but the rubber shock pad is overlaid with a spikeresistant flexible surface rather than a sand-filled carpet.

The macadam of the invention, however, is imact absorbing to thestandards of BS 7044, BS 7188, BS 5696 and all relevant govering bodiesof sport that specify these or similar tests. Accordingly the macadam ofthe invention can be used to replace both the macadam base and the shockpad of a conventional synthetic sports pitch, simply being overlaid withthe sand filled carpet. Likewise the macadam of the invention can beused to replace both the macadam and the rubber/resin base layer of aconventional synthetic athletics track, simply being overlaid with aspike resistant flexible surface.

EXAMPLE

A typical mix consists of

Binder 7.0% rubber grading

10 mm Aggregate 24.0% 100% passing 10.0 mm

6 mm Aggregate 10.5% 35% passing 6.3 mm

Sand 21.3% 0% pausing 4.0 mm

Rubber 35.0%

Filler 1.9%

The binder comprises 7±1% unbranched styrene butadiene styrene blockcopolymer blended with 93±1% of a compatible bitumen having thefollowing properties.

penetration at 25° C. 90±20 dmn

Softening point 85±10° C.

Storage stability index 75° C.

This mix is based on usage within the United Kingdom at an averageambient temperature of 15° C. Mix ratios may be adapted for useelsewhere, in a manner known to those skilled in the art, to compensatefor the ambient temperature of the location.

What is claimed is:
 1. An impact absorbing macadam comprisingparticulate rubber having a particle size of up to 40 mm, an aggregatehaving a particle size of up to 40 mm and a polymer modified bituminousbinder, wherein the rubber comprises from 10 to 75% of the macadam dryweight, the aggregate comprises from 25 to 90% of the macadam dryweight, and the binder comprises from 5 to 9% of the macadam dry weight.2. The macadam according to claim 1 having voids and a void volume offrom 5 to 25%.
 3. The macadam according to claim 2 having a void volumeof from 15 to 25% wherein the voids are interconnected.
 4. The macadamaccording to claim 2 having an unconnected void volume from 5 to 20%wherein each void is isolated from adjacent voids.
 5. The macadamaccording to claim 1 containing from 25 to 45% by weight of theparticulate rubber.
 6. The macadam according to claim 1 in which theparticulate rubber has a particle size of up to 15 mm.
 7. The macadamaccording to claim 1 containing from 45 to 65% by weight of theaggregate.
 8. The macadam according to claim 1 in which the aggregatehas a particle size of up to 15 mm.
 9. The macadam according to claim 1containing from 6.5 to 7.5% by weight of the polymer modified bituminousbinder.
 10. The macadam according to claim 1 in which the polymermodified bituminous binder comprises an unbranched styrene butadienestyrene block copolymer and a bitumen compatible therewith.
 11. Themacadam according to claim 1 in which the polymer modified bituminousbinder comprises a styrene butadiene rubber and a bitumen compatibletherewith.
 12. The macadam according to claim 1 in which the polymermodified bituminous binder contains from 6 to 8% by weight of thepolymer.
 13. A roadway formed from an impact absorbing macadamcomprising particulate rubber having a particle size of up to 40 mm, anaggregate having a particle size of up to 40 mm and a polymer modifiedbituminous binder, wherein the rubber comprises from 10 to 75% of themacadam dry weight, the aggregate comprises from 25 to 90% of themacadam dry weight, and the binder comprises from 5 to 9% of the macadamdry weight.
 14. A sports pitch having a base formed from an impactabsorbing macadam comprising particulate rubber having a particle sizeof up to 40 mm, an aggregate having a particle size of up to 40 mm and apolymer modified bituminous binder, wherein the rubber comprises from 10to 75% of the macadam dry weight, the aggregate comprises from 25 to 90%of the macadam dry weight, and the binder comprises from 5 to 9% of themacadam dry weight wherein the macadam is overlaid with a syntheticcarpet, the pile of which is filled with sand.
 15. An athletics trackcomprising a base formed from an impact absorbing macadam comprisingparticulate rubber having a particle size of up to 40 mm, an aggregatehaving a particle size of up to 40 mm and a polymer modified bituminousbinder, wherein the rubber comprises from 10 to 75% of the macadam dryweight, the aggregate comprises from 25 to 90% of the macadam dryweight, and the binder comprises from 5 to 9% of the macadam dry weightwherein the macadam is overlaid with a spike resistant flexible surface.